Method and apparatus for corrosion protection



Nov. 5, 1968 W. P. BANKS ET AL METHOD AND APPARATUS FOR CORROSIONPROTECTION Filed Aug. 13, 1965 5 Sheets-She??I 1 Nov. 5, 1968 W. P.BANKS ET AL METHOD AND APPARATUS FOR CORRO'SION PROTECTION Filed Aug.13. 1965 5 Sheets-Sheet 2 JMW fw Nov. 5, 1968 w. P. BANKS ET AL3,409,526

METHOD AND APPARATUS FOR CORROSION PROTECTION mLmHFUIrI NILmHrP Nov. 5,1968 w. P. BANKS ET AL 3,409,525

METHOD AND APPARATUS FOR CORROSION PROTECTION Filed Aug. 13, 1965 I 5Sheets-Sheet 4 +/ooo 4o ,v0/5.45 o 0.0/ 0./ /.o /o /oo auer-,vr Deus/ryA44/sq. /A/cH I'LE.-E

METHOD AND APPARATUS PoR coRRosIoN PROTECTION Filed Aug. 13, 1955 Nov.5, 1968 w. P. BANKS ET AL 5 Sheets-#Sheet 5 HQIMII '.5- 5 s H @www T A G.r Nu@ .Y E o F VBC /N mM/a M .AN U um@ A #5% y A B United StatesPatenti() The present invention relates to an'improved' method l andapparatus for controlled potential cor'osio'rr protectionof metallicbodies, and morepa'ticularl'y, but rnot'by way of limitation, theinvention is concerned with controlling the corrosion rate of certaintypes of special corrosion reaction wherein both anodic andcathodiccurrents are corrosive solution.

As it is well known in the art of corrosion'v control,the corrosion ofmany metals may be prevented or-largely 'reencountered at the interfaceof a metallic vessel and-'its duced by inducing passivity in the 'metalby anodic pol'ari- `zation techniques. Recently, -amethod and apparatusyfor corrosion prevention by means of anodic polarization has beendeveloped wherein a metallic 'specimenfsuh as -a vessel to be protectedagainst corrosion'fby Va' chemical contained therein,l is anodicallypolarized with respect to an inert'electrode suspended in the corrosivel'electrolyte in the vessel. An electrical current ispassed betweenth'emetallic vessel and the inert cathode in amanner such as to maintain theelectrical potential of the vessel in' a-so'-A called passive region,that is,` a potential range in which the rate of corrosion of the vesselis minimized'. The mag nitude of the current which is applied betweenthe vessel and the inert cathode is at all'times determined by' thepotential of the metallic vessel, with the fcurr'ent being varied asnecessary in orderto'rnaintain the potentialof the vessel in the regionof passivity. f

' 'In the 'commercial systems which havel been developed for protectinga metallic member through anodic polari'- zation, a reference electrodeof constant potential Vis placed in electrical'communication with thecorrosive-electrolyte contacting the metallic memberj'andv the potentialdifference between such refeernce electrode and the 'metallic member isconstantly monitored. This potential difference, which may be termedthereference potential, is con:- stantly compard electricallyA with aysecond potential which is derived as a control potential. The controlpoten tial is the potential diierence which, according tolpre-idetermined polarization curve data, must exist between the metallicmember and the reference electrode'if the vessel is to be maintained inafpassivestate. Thecontinuous electrical comparison of thereferencepotential 'with-'the control potential resultsA in the continuousAgeneration `of an error voltage which provides a signal usabletoincrease or decrease the amountof currentpassed between the metallicmember and the inert cathode'suspended inthe electrolyte.. In otherwords, the potential of -theI-metalli'c member is constantlymonitored,and the anodic corrosioncontrol system functions to develop anerror signal in the manner described to constantly maintain the metallicmember at a passive potential.- Y .I l

The above types of anodic passivation system have been developed in avariety of equipments for specific corrosion control applications.Generally speaking, anodic passivation systems lare employed inmetal-electrolyte 3,409,526 Patented Nov. 5,l 1968 ICC 'combinations"that include Zhighly conductive solutions which'exhibit the" propertyof es'tahlishir'izgl a passive range with 'the metal' in questioniAHence, Laiiotiic passiv'ati'm` is particularly adaptable toiusaige iniroii'or stainlessvsteel storage or processing vessels which contain acorrosive, electrolyte solution',`` such as, sulphiirieiacid,"phosphoricacid; sodium hydroxide, lithium hydroxide, 'alnninur'nsulphatenitic"acid and many other corrosive liquids which 't'ndVextensivecornmer'c'zial employ. *i 'f if Itlias now beeniidiscoveredthat 'there lis another class of corsionrea'ctiti'nsv which exhibit asomewhatfdiiee'ritdntliavioi' at the lrnetal-electrolyte:interfaceduring cor'- rosion; That "is,'A when the'fm'etal-electrolyte systemfisin its paesive range, both vanodic and cathodic current'sfare generatedwithin the passive potential range of the metal. This specialpolarization behavior was noted in systems of Carpenter 20 Steelcontaining a solution of sulphuric acid and'zinc. Other additionalmetalfelectrolyte systems have also been found to exhibit the specialpolarization behavior; for example, systems of stainless steelcontaining hot, concentrated alum solutions, anda pulp`digester systemconsisting of a carbon steel vessel'tzontining alkaline sulphide cookingliquor. Still other metal-electrolyte 4systems will exhibit the specialpolarization behavior and would thus be suitable subjects for the employof the present invention. l

The present invention contemplates a method and ap? paratus forcontrolled potential corrosion protection in metal-electrolyte systemswhich exhibit special polarization behavior wherein a region of cathodiccurrentactiw. ity as well as a region of anodic current activity is.present at the more noble, non-corroding metal potentials. The .methodrequires that polarization current of either polarity be available forapplication to the metal vessel', depending upon the 'potential of thevesselnand its corresponding anodic or cathodiccurrent requirementsasdetermined from thechi'acteristic E versusI curve for the particularmetal-electrolyte system. The 'apparatus of the inventionprovideselectrical connections so that current may be passed from asuitable-.bi-polar power source through the anode, the electrolyte andan axiliary electrode immersed in the electrolyte. The control equipmentis suchy that "the polarity of the current applied from the externalpower source can b e reversed in accordance with the current densityrequirement as dictated by the instantaneous polarization current atrthemetal-electrolyte interface, that is, whether anodic 0r cathodic currentow. A reference electrode is also immersed in the corrosive solutionsuch that it may continually monitor the potential of the vessel forcomparison with a preset control point voltage to. provide an indicationwhereby thepower supply can be continually controlled as to the polarityof its application betwen the vessel andauxiliary electrode. iv It is anobject of .the present invention to provide a method of corrosionprotection for metal-electrolyte systems which exhibit a specialpola-rization cur-rent b ehavior. f

It is Ianother object of the present invention `to provide a corrosionprotection method for a class of metal-electrolyte systems which includeCarpenter 20 Steel-Sulphuric -acid systems, steel-alum systems, anddigesters, particularly of-the carbon steel-alkaline .sulphide.1iquortype I t It is further an object of the present invention. to provide amethod of corrosion protection for metal-electro-A lyte systems whichexhibit both anodic and cathodic current density regions within the morenoble, non-corroding range of vessel potentials.

It is still another object of the present invention to provide apparatusfor monitoring vessel potential and controlling the polarity of voltageoutput from a power supply to the vessel system in accordance with thepotential indication.

It is yet another object of the invention to provide apparatus forgenerating control signals in response to predetermined vessel potentialvalues for the purpose of controlling the polarity of application of adirect current power supply between a metallic vessel and an auxiliaryelectrode.

Finally, it is an object of the present invention to provide corrosionprotection for a class of metal-electrolyte systems exhibiting specialpolarization current behavior wherein the potential of the vessel can becontinually monitored with respect to a reference control voltage toprovide a further voltage indication which controls a power source toapply either positive or negative polarity of current to the vessel,depending upon whether there is anodic or cathodic current activity atthat particular potential of the vessel.

Other objects and advantages of the invention will be evident from thefollowing description when read in conjunction with the accompanyingdrawings which illustrate the invention.

In the drawings:

FIG. 1 illustrates an exemplary polarization curve of the usual typewhich is characterized by the presence of anodic current density withinthe entire passive range;

FIG. 2 illustrates a polarization curve which shows special polarizationcharacteristics;

FIG. 3 is a block diagram of an equipment suitable for carrying out thepresent method;

FIG. 4 is a polarization curve for a Carpenter 20 Steel- Sulphuricacid/zinc system which exhibits special polarization behavior;

FIGS. 5, 6, 7, 8 and 9 show a family of polarization curves for a 1020mild steel-alkaline sulphide liquor system at the respectivetemperatures of 150, 207, 250, 300 and 350 Fahrenheit;

FIG. l is one form' of apparatus suitable for carrying out the presentmethod; and

FIG. 1,1 is an alternative form of apparatus for carrying out themethod.

FIG. 1 illustrates a typical anodic polarization curve depicting theanodic current behavior of a stainless steel vessel containing sulphuricacid. This is the usual form of E vs. I characteristic curve of the typesuitable for anodic polarization techniques as previously ascribed tothe prior art. It should be noted that the current is anodic through theenti-re range of vessel potentials, that is, from the active potentialsthrough the more noble potentials. The ordinate 10 is graduated inmillivolts and indicates the metal potential as derived from a saturatedcalomel electrode in contact with the corrosive solution. The abscissais graduated in amperes and indicates the relative anodic cur-rent owbetween the vessel (anode) and a cathode immersed in the electrolyticsolution.

The value of about +600 millivolts found at point 13 (zero amperescurrent) of the current density curve 14 is defined as the naturalcorrosion potential. This potential may be shifted 'toward the noble endof the electromotive series by applying current through the electrolytebetween the stainless steeel vessel (anode) and the inert cathodeelectrode. The amount of current required to shift the potential in themore noble direction increases along the curve 14 until a point 16, theFlade arrest potential, is reached. At the Flade arrest potential, theamount of current required to shift the potential decreases rapidly andremains at a very low value until the transpassive region 18 is reached.

The potential range above the Flade point 16. from about 100 millivoltsup to 600 millivolts, the natural corrosion potential, is termed theactive region. Corrosion is greatly accelerated in this portion of thecurve. Below the Flade potential 16 and within the bracketed portion 20,there is what is termed the passive region of the curve whereincorrosion is practically halted, as shown by the very small lrelativevalue of anodic current. When the vessel potential (to the Ireferenceelectrode) showsistill further decrease, the condition passes into thetranspassive region wherein high current flow is again'v 'present andcorrosion protection is lost. The bracketed portion 22 denotes a desiredrange of passivity at which effective cor- -rosion protection can becarried out. An anodic protection system would function to maintain thepotential of the containing vessel (with respect to a calomel referenceelectrode) within the potential limits of the region 22; however, thepotential limits as set by the larger bracket 20 could also be used togive reasonably good corrosion protection.

The method and apparatus of the present invention is concerned with aspecial polarization behavior as exhibited by certain metal-electrolytesystems, an example of which is shown by way of FIG. 2. FIG. 2 shows asemilog E vs. I plot, polarization curve 24, which was derivedfor 304stainless steel in contact with hot, concentrated alum solutioncontaining 14 percent aluminum 0xide (A1203) lat 220 F. The potentialmeasurements were taken by means of a saturated calomel electrode inelectrode in electrochemical contact with the hot alum solution at F.The ordinate 26 is calibrated `in millivolts to show the potentialdifference between the calomel standard electrode and the metallicvessel and the .abscissa 28 is calibrated logarithmically to show thecurrent density of the metal-electrolyte system in miliamperes persquare inch of the metallic vessel of the system. Further, although 304stainless steel is specified here, it should be understood that similarbehavior is evident with other stainless steels or alloys having a highnickel or chromium content.

At the potentials more active than the natural corrosion point 30 thereis a primary cathodic current region 31 wherein the metal is activelycorroding. As current is applied to the system to reduce the potentialto a more noble or more stable value, a very high initial anodic currentis required to move the vessel potential from the natural corrosionpoint 30 (about +300 millivolts) through the Flade arrest point 32. Withfurther potential decrease the anodic current density drops abruptly andbecomes cathodic in character as shown by the secondary cathodic currentregion 34. With still further decrease in the vessel potential, thepolarization curve indicates that anodic current density is once againpresent (below about -200 millivolts) which traverses a short peak 36,of no particular consequence, and finally goes to the transpassivecondition, indicated by region 38 of the polarization curve 24.

It should be understood that the potential of the reference voltage inFIGS. 2, 4 and 5 through 9 are opposite in polarity to values whichwould be shown under the standard convention as adopted by theInternational Union of Chemistry and Applied Physics. This conventionmaintains that potentials moving from active to noble should progressfrom negative to more positive potentials. The potentials under theconvention are measured by a reference electrode which is connected tothe common side of the measuring circuit. However, the potentials asshown and described herein are measured with respect to the common sideof the measuring circuit, an integral part of the system, as Will bedescribed.

The method disclosed herein enables corrosion protection ofmetal-electrolyte systems which exhibit such special polarizationbehavior by first applying anodic current to take the system through theFlade potential 32 and thereafter utilizing a reference potential ass'ensed by a reference electrode for controlling the amount and polarityof current application to the system. Thus, the require-` ments for'corrosion protection may require either cath'-r odic or anodic currentflowwithin thecorrosioniprotection range o f more noble metalpotentials. That is, the range of metal potentials more noble than' theFlade arrest potential at which the current density between the metalsystemis maintained stable `and non-corroding. Y

In applying the method to a metallic vessel-corrosive solution system,it is necessary to control the vessel potential continuously byapplication of the proper polarity of current through the solution. Therequired manner of current lapplication will be apparent from apolarization curve which is established for the particular metal vandsolutionf high value of anodic current is first appliedto s'hiftthevvessel potential to a value slightlyrbelow the Flade arrestpotentialand, in cases of special polarization behavior, this will be anon-corrosive Acathodic current region. The vessel-solution system canthen be maintained within that region by controlled application ofreversed and electrolyte isl at a low value and the metal-electrolytelor cathodic current through the solution.

if 'In a certain few metalLelec'trolyte systems which exhibit thespecial polarization behavior, it has been found that existing anodicpassivation equipment can be used to prevent'cor'rosion to some extent.For example, equipment such'as that disclosed in U.S. Patent No.3,127,1337, entitled Anodic Passivation System; issued to Conger et al.and assigned to the present assignee.

' This equipment'can be applied in its usual manner to lower thepotential of a metallic vessel through the Flade arrest point and intothe secondary cathodic current re gion whereupon anodic currentapplication can be ceased. In'these few metal-electrolyte systems thevessel potential will remain in the secondary cathodic currentregion forappreciable periods of time due to the inherent -nature of the system toseek a stable, non-Vcorroding potential. However, this type ofAprotection is limited to a lfew applications and does not have theversatility required for reliable controlled potential corrosionprotection of the large majority of the metal-electrolyte systems whichexhibit special polarization behavior.

FIG. 3 shows'a block diagram of a corrosion protection equipment whichhas the"required versatility to carry'out the method set forth herein.The vessel 50, a control electrode in the'metal-electrolyte system,contains a corrosive solutionv 52, and an auxiliary electrode 54 anda'reference electrode 56 are immersed in contact with the corrosivesoltionSZ. The auxiliary electrode54 isformed of a material which isnon-reactive with the corrosive solution, preferably platinum or anotherof the inert metals. The reference electrode 56 consists of a standard'half-cell, many types of which are employed in the corrosion lcontrolart, which may' beplaced in electrochemical contact with the solution 52through an agar or potassium chloride (KCl) salt bridge juncture. o i lA potential vcontrol circuit 58 supplies a predetermined control voltageVcon the lead 60 to the voltage comparator or differential amplifier 62.Similarly, the reference electrode 56 supplies its voltage indication,on lead 64 'to voltage comparator 62 while the vessel elec-trodeconnection 66 supplies the common vessel potential to the voltagecomparator 62. The voltage comparator 62 senses the potentialditerencebetween the voltage on lead 64 and the common vessel voltage onlead 66 to derive a relative vessel potential, the reference voltage Vr.This relative vessel potential is then compared with the controlpotential Vc on lead 60 which is preset at a value derived from thepolarization curve of the particular metalelectrolytesystem inconjunction with the particular halfcell 56 which is employed. Thus,thev voltage comparator 62 derives an output signal on lead 68 which isindicativev of the amount and polarity of potential difference betweenthe vessel 50 and the preset potential control 58. There are variousforms of voltage comparator 62 which could be employed in the system andthese will ber further described in connection with specificapparatus.

"The difference or error control voltage VVeron lead 68 is thenamplified in a` control amplilieri70 and applied on lead 72 to a currentregulator circuit 74.W"A.high current power 'supply 76, providingbothfnegative and positive output currentl'sources, has itscommon'terlmnal 'connected via a lead 78 to` lead 66 and to the'metallic vessel 50; Ajnegativ'e current output leadv `80 and a positivecurrent'output lead 82 rare then connected through the current regulatorcircuit 74 which has a'single output lead 84 connected to theauxiliary'electrode 54. The current regulator circuit 74 isV responsivetothe control signal on lead 72 from'control amplifier 70 `to switoheither the negative current' (lead 80) ofthe positive current (lead 82)for connection to the auxiliary "electrode lead` 84. The currentregulator circuit l7 4 also `re sponds proportionally tothe"control"signal onlead 72 to lvary the' amountofcurrentpa'ssingthrough 4the'lswitch connection to theauxiliary electrodelead`84. A'complete description of suitable L"current regulato'circuitryi`s` to be furthendescribed in connection""\vith specific ap; paratus. LThere are various other metal-electrolyte systems which show suchspecial polarization characteristics that'the present method ofcontrolled potential corrosion protection is required. Polarizationcurves for each of the metalelectrolyte 'systems exhibits the expectedswing from'the primary cathodic current region through the naturalcorrosion point to the Flade arrest point and then through a sharpdecrease of anodic current. With further increase towards the more noblevessel potential, the sys` tems pass through a region of secondarycathodic current density and then back to the anodic vcurrent conditionand finally into the transpassive region of anodic current density. Thefollowing curves of FIGS. 4 through 9' are shown in a different formfrom FIG. 2 with both the anodic and cathodic' current density patternsbeing shown on the same side of the ordinate. It should be understoodhowever that the curves are the same type as shown in FIG. 2 and thevariation is merely a space-saving measure. L

FIG. 4 shows the graph ,or polarization curve'for anothermetal-electrolyte system which -inds extensive commercial employ andwhich exhibits special polarization behavior. The curve indicates thecurrent density versus the vessel potential for Carpenter 20 steel in200 grams per liter sulphuric acid and 65 grams per literzinc at 65 C.The vessel potential indicated on the ordinate is indicated with respectto a saturated calomel half-cell as the reference electrode and thereference electrode was maintained at 80 Fahrenheit in electrochemicalcontact'with the sulphuric acid/zinc solution. The system exhibitsl theexpected primary cathodic current region 90 at the -most activepotentials withv av rapid transition through the natural corrosive point92 at about '200 millivolts vessel potential. Further decrease of thevessel potential indicates that the current density increases rapidly tothe Flade arrest point 94, at about 0 millivolts vessel potential; andthen falls rapidly to the secondary cathodic current region 96. At avessel potential of about -700 millivolts the current density once againbecomes anodic and thereafter it passes into the transpassive region98.'

Inspection of the curve indicates that ay secondary cathodiccurrent'region 96, from about 200 millivolts to -700millivolts,"constitutes a range of potential values more noble than theFlade arrest point 94 wherein the vessel is in a stable, non-corrosivecondition. Thus, it would be desirable to employ the controlledpotential protection system to maintain the vessel potentialY between-200 and -700 millivolts and to prevent corrosion of the Carpenter 20steel vessel.

The 500 millivolt range of the cathodic current region 96 is more thanadequate since the protection system at most only requires about 50 to100|mil1ivolts of correc-l tion range about a control point. Thus, theoperator may choose an optimum value at which to set the control pointwithin the secondary cathodic current range. This will depend upon theshape and amount of current density in the secondary cathodic currentloop of each particular metal-electrolyte system. Similar behavior ofspecial nature is exhibited by many metal-electrolyte systems whichinclude zinc ions in solution and thus the present method of corrosionprotection is particularly applicable.

FIGS. through 9 illustrate a family of polarization curves for ametal-electrolyte system which finds extensive industrial usage. Thecurves indicate the current density and therefore the corrosion effectsof alkaline sulphide liquors in pulp digesters. Corrosion has been agreat problem with all pulp digesters having practical utility and theequipment cost and replacement expenses necessitate the application ofan elective controlled potential protection method. In the operation ofa pulp digester, the metallic digester vessel is charged with the pulpand alkaline sulphide liquor, and the batch process is then carried onfor a number of hours at a high temperature of about 350 F. Once thedigester and its contents is up to operating temperature, at about 350F., there is little or no danger of corrosion, at least not such aswould detinitely require corrosion protection. However, corrosion isextremely high during the heating period and this is where the problemarises. The following family of curves will illustrate the variations ofbehavior of the pulp digester metal-electrolyte system at the varioustemperatures of operation and this consideration accentuates the needfor a flexible and accurate controlled potential protection system.

FIG. 5 shows a first polarization curve for a pulp digester system at150 F. which is somewhat below the starting temperature of a typicalpulp digester process. Since the usual or average liquid charge to analkaline sulphide pulp digester can consist of volumes of white (active)and black (spent) liquors as dilute as 50/50 volume percent, thepolarization curves were determined for a 50/ 50 volume percent mixtureof white and black liquors. The active or charging liquor isapproximately 4.7 weight percent sodium hydroxide (NaOH) and 1.6 weightpercent sodium sulphide (NazS). The metal of the system is 1020 mildsteel, one of the carbon steel materials commonly used in pulp digestervessels. Also, in each of the curves of FIGS. 5 through 9, the vessel ormetal potential is indicated with respect to a saturated calomelhalf-cell at 80 F. (thus it is in heat insulative, electrochemicalcontact with the charged solution.

As shown in FIG. 5, the primary cathodic current region 100 is evidentat the more active or highly corrosive vessel potentials. Then the curvedrops to zero current density at about +950 millivolts, the naturalcorrosion potential 102. The current density then becomes anodic andrises rapidly to the Flade arrest -point 104 and at a vessel potentialslightly below 900 millivolts the anodic current density ceases and theexchange reverses to a cathodic current density as outlined by thesecondary cathodic current region 106. Thus, there is a cathodic currentbetween vessel potentials from about +875 millivolts inthe nobledirection to about +800 millivolts and this would constitute a desirablerange in which to set a control point of the controlled potentialprotection apparatus. As the digester vessel is made more noble, thatis, the vessel potential becomes more negative than the +800 millivolts,the current density becomes anodic and traverses into the transpassiveregion 108.

FIG. 6 shows the 1020 mild steel-alkaline sulphide polarization curve at207 F. as it would exist shortly after a digestion process was begun.This curve shows the similar current density characteristics, that is, aprimary cathodic current region 110 through a natural corrosion point112 and then through the anodic current region and Flade arrest point114 to the secondary cathodic current region 116, the desired stablecontrol point. The secondary current region 116 still shows a vesselpotential range between about 800 and 900 millivolts wherein veryeffective control can be maintained and then at more noble potentialsthe current density would become anodic into the transpassive region 118of the curve. This curve is highly similar to'the 150 F. curve (FIG. 5)but it should Vbe noted that the Flade arrest point 114 is at a slightlyhigher vessel potential as is the natural corrosion point 112.

FIG. 7 illustrates the curve with the same digester materials after theoperating temperature has reached 250 F. The general form of the curveis still the same although the various sectors of cathodic and anodiccurrent are changed somewhat in shape. The secondary cathodic currentregion 120 still provides a reasonably wide potential range between thedigester potential vessels of about 750 to 850 millivolts. This is morethan ample control voltage range and the controlled potential protectionsystem can be set to maintain the digester in a noncorroding condition.FIG. 8 shows the same curve after the reaction temperature has reached300 F. Again the overall shape of the curve is changed, however, thegeneral form is similar as there still remains a Flade arrest point 122as well as a region of secondary cathodic current 124. At 300 F. thesecondary cathodic current region 124 shows the presence of cathodiccurrent over an increased potential range of the metallic vessel. Thevessel -would be maintained in a stable, non-corrosive condition as long`as its potential was in the range from about +875 millivolts to about1050 millivolts. The potential control point of a protection systemcould be set anywhere within this 175 millivolt spread to effectcorrosion protection.

FIG. 9 shows the polarization curve for the digester system after it hasreached 350 F., its cooking or processing temperature. It should benoted here that there is no typical anodic Flade arrest point, merely aprimary cathodio current region 126, a natural metal-electrolytepotential 128 at about 925 millivolts which is largely noncorroding, andan anodic current region 130 which extends into the transpassivecondition of the metal polarization. -In 4this the cooking condition,the pulp digester system exhibits little or no corrosion; hence,corrosion protection is not necessary once the pulp digester vessel hasattained its processing heat of 350 F. As previously stated however, thegreat and excessive amount of corrosion takes place during the timebetween the initial charge of the cooking liquor into the digestervessel :at much lower temperatures and up to the time when theprocessing temperature of 350 F. is reached. Thus, the controlledpotential corrosion protection system is employed during this time ofheat-up to combat the excessive corrosion taking place in that period.It is important also to note that a corrosion protection system for suchpulp digestion processes must exhibit certain flexibility since thevessel potential ranges of secondary cathodic current are continuallyvarying as the temperature climbs to the final processing level.

The following table lists the corrosion rates and other pertinent datafor 1020 mild steel in alkaline sulphide liquor. Comparison of thevarious results will indicate the severe corrosion which takes placeduring the heat-up period of the process. Observation of the resultsobtained at 350 F. or the cooking temperature will reveal the contrastin data and the relative corrosive stability of the metal-electrolytesystem at the highest temperature.

The test data of the table (below) was compiled with laboratoryequipment using 1020 mild steel coupons immersed in a 50/50 volumepercent mixture of white and black alkaline sulphide liquors at thevarious temperatures as listed. The reference potentials, taken withrespect to the 1020 mild steel coupons, are the potentials for asaturated calomel half-cell; hence, they are relative potential valuesfor the mild steel coupons in each instance. The unprotected couponswere periodically activated by passing a few milliamperes of cathodiccurrentthrough'the'coupon 4at the" beginning of the test, and thenatintervals throughout-the test, it necessary, -to keep the couponpotential lat a stable corroding value.

rosive solution 142. The power application lead 152 is connected to theauxiliary electrode 150 from a'power source to be described. Thereference electrode 144 is connected via a lead 154 to a potentialcontrol circuit or Liquid Phase` Unprog Corrosion Rate, Number tectedPolarization Current Temperature, Pressure, Milli-inches per year ofActi- `Coupon Potential, Density, Fahrenheit p.s.i.g. w vationsPotential, Millivolts Milliamperes/ v Unpro- Polarized Millivolts l sq.inch tected f f 147 557 0.0 3 y 950 840 0.015 Cathodic. 208 86. 0.04 1980 850 0.083 Cathodic. 250 31 0.0 2 l 1,085 875 0.05 Cathodic. 300 50161 0.0 9 1,145 975 0.22 Cathodic. Y 350 100' 423 0.0 15 f 1, 145 8750.07 Anodic. 350 100 0 0.0 0 885 845 0.18 Anodic.

The activated, unprotected coupons corroded at a high rate, rangingfrom31 to 557 milli-inches .-per yearA in the temperatureinterval between147 and 350 F. The tendency of the .unprotected coupons t-ospontaneously shift to the stable non-corroding potential lvalue.increases markedly at'the higher. temperatures.l This tendency isconsistent with conclusions which can be drawnv from the respectivecurves of.FIGS. 5 through 9.= s v The coupon corrosion rates for thepolarized or protected phase of the tests were zero throughout the.entire temperature range from 147i to 350 F. At the tempera ture..ybelow'350 F., the coupons were polarized at potentials corresponding tothe secondary cathodic current region of the polarization curve. At 350Fahrenheit, the secondary cathodic region is absent from -thepolarization curve,.and the coupons were maintained at a-potentialcorresponding'to a low anodic current density. These results prove thevalue of the present method of controlled potential-corrosionprotectionfor metal-electrolyte systems exhibiting special polarization behavior.y

' APPARATUS There are several forms of apparatus which have beenconstructed for use in controlled potential corrosion protection. Somealternative forms which :are suitable for performing the present methodare set forth hereinafter.' As will become apparent, thediierentequipments exhibit certain advantages and disadvantages -forparticular corrosion protection applications due to their inherentcircuit characteristics andpower 4handling capabilities. v Oneembodirnent'of the controlled potential-corrosion protection apparatusis illustrated in FIG. 10. The .appaf ratus mightbe employedwith any ofthe metal-electrolyte systems exhibiting the special polarizationbehavior as previously discussed. For example, the metal-electrolyte systern may comprise the materials as set forth with regard to FIG. 4. Thatis, the metallic vessel 140 would be formedl other suitable types suchas a silver-silver chloride cell,

copper-copper sulfate cell, molybdenummolybdenum oxf ide, and so forth;however, the polarization curve (FIG. 4) requires the use of calomelsince it was compiled from such reference.

The reference electr-ode 144 is shown communicating electrochemicallywith the corrosive solution 142 through a suitable electrolytic bridge146 to preventthe reference voltage cell from coming into direct contactwith the solution 142. While an electrolytic bridge 146 is showncommunicating with the solution 142, it should be understood by thoseskilled in the art that various well-known equivalents may be used toionically couple the reference electrode 144 to the corrosive solution142.

An auxiliary electrode 150, formed from an inert material, is suitablymaintained in direct contact with the cor- Y 140 to the input of controlamplifier 166 and a-parallel would require only normal skill in the art.

power lead 170 is applied to the common or midpotential point of a powersupply 172. The control amplifier 166 may be any of several well-known`types of error amplifier which are used in the corrosion art. 'Ihe onlyrequirements being that the amplifier have high input impedance,l thatit be reasonably stable and free from drift due to' temperature, andthat it should have high sensitivity since the totalV range of controlsignals must be effected from relatively small input signal strength.The power supply 172 may be one of the conventional types of powersupplywhich recties A-C-input power and provides both a negative and apositive output D-C voltage with respect to a common or mid-potential.Such power supply equipment is a matter of choice and theproperselection tive output terminal of power supply 172 is connectedvia lead 186 tothe collector 188 of an NPN-type transistor 190. Thenegative emitter 192 is also connected through the power lead 152 to theauxiliary electrode 150 and the base 194 also receives control voltageon lead 196 from the output of control amplifier 166.

'The transistors 178 and 190 being connected in parallel,l but foropposite conduction, serve as a proportional current regulation circuitto apply varying amounts of either cathodic or anodic current flowthrough the load circuit; consisting of the electrochemical cell formedby the metallic vessel 140, the corrosive solution 142, and theauxiliary electrode 150. When the output control voltage on lead 184from control amplier 166 is more negative than a predetermined midvalue, the transistor 178 will conduct such that the negative terminalof power supply' 172-is in the circuit with the auxiliary electrode150'. In this condition, the common lead 170 carries a more posi- E tivepotential and the current flow through the corrosive 13 lead 216 to theimpedanceconverter 224. The half-ce reference electrode 206 is alsoconnected on `lead 226` to the input of impedance converter 224.Theimpedancecorrverter 224 is a well-known typeof amplifier-stage whichprovides an output voltage indicativek ofY the reference potential onlead 226 as taken with respect to the metallic vessel 200. Further, theimpedance converter stages v224 are preferably employed'with adequatefeedback to main, tain unity gain at the output. Thus, the actualpotential Vrof the metallic vessel-200 is indicated on the outputpresent on lead 228 from the' t impedance converter 224. A potentialcontrol` circuit 230,- similar to the potential control 156 of FIG. 10,provides a control point or set point voltage on lcadf232 for.application to the power control circuitry as lwill zbeldescribed;

The potential control 230 may take-any of `various forms .f

of variable direct current vSource andthe elaboration .of this circuitwill` depend upon the accuracy lrequired'in the particularcorrosionprotection application.VV z a f The circuitry which isemployedfor detectingdifference between'the reference voltage Vrzonleadv 228 and the control point voltage Vc on leadsA 230 consists ofoppositely oriented, balanced pairs of PNP-type transistors. A firstbalanced pair,`transistors 232 andv 23-4. are`con= nected in parallelthrough asuitable dropping resistor: 236 to the positive power supplyterminal 238. he collector of transistor 232 is connected through a`resistancel240 to the negative power supply terminal .242 andthecollector of transistor 234 is connected through a capacitor 244 tothe negative power supply terminal242. The-ca"- pacitor 244is atiming-capacitor for a'relaxation oscillator circuit which operates tochargeiin conjunction with the transistor 234 conduction as will bedescribed. fA second balanced pair, transistors 246 and.248v areconnected in identical configuration. Thatis, therespective emitters areconnected together and lead through a resistance '250 to the positiveterminal 238A and input-transistor 246has its collector tied throughresistance 252 to ythe-negative terminal 254 while the collector'ofoutput transistor 248 is connected through another timingv capacitor 256tothe negative terminal 254. f Y

the power supply 214 is what may be termed phase con; trol. That is, thecircuitry develops a pulse-type control signal having a phase variationproportional to .the'fdif-v ference between the potential of metallic-ve`sselr200- and the control point potential Vc. 'I'he.metallic.vesselpoten-y tial or reference voltage Vr present on leads 228-is ap plied tothe base of output transistor: 234l of one balanced pair while it isalso applied to the base of-input transistor 246 of the other balancedpair. In the same manner, the control point voltage Vc present on leads:230v is v'applied to the base of the output transistor 248.0f 'thesecond bal-v anced pair while being applied to the base ofinput-transistor 232 of the first balanced pair. Thus, anyI differencevoltage between the reference voltage VrandY the control pointvoltage-Vc will cause opposite conductionasbetween the two balancedpairs of transistors,` transistors 232 and 234 and transistors 246 and248. Therefore, the outputs from the balanced pairs of transistors willalways have an opposite relationship.

A first output taken from the collector of transistor 234 on a lead 258is applied to the emitter of a unijunction transistor 260, a relaxationoscillator element. The base number one of the unijunction transistor260 is conducted through the primary 262 of transformerl 264 to thenegative power supply terminal 242. The base number two of unijunctiontransistor 260 is connected through a .suitable resistance 266 to thesystem ground. Similarly, the output from the second balanced pair oftransistors 246 and 248 is taken from the collector of transistor 248via lead 268 to the emitter of a unijunction transistor 270, also arelaxation oscillator element. The base ynumber one of transistor 270 isconnected throughv the primary 272 of a transformer 274 to the negativeypower supply terminal 254. The basel number two of transistor 27 0 isconnected through aresistance 276 to thefsystem ground; thusproviding aunijunctiontransistor circuit'which is identical to the previouslydescribed unijunctiontransistor 260 circuit. However, the respectiveemitters will be alternately controlled as will be described' inconnection with the operation ofthe apparatus. AThe system isenergizedby ,a full wave', `unfiltere power supply 278. An A-C lineinput source'280 supplies line voltage on leads 282 to the transformer284 and secondary 286. Transformer 284 would beV any ofthe well-knowntypes which provide anoutput voltage value suitable for energizingtransistorized equipment. The ends ofthe secondarywinding 286 areconnected to the Vrespective cathodes ofsuitable -rect-ifiers 288 and290 and their respective' anodes 'are tied together at a terminal point292 and negative supply lead 294. The center tap 296 is'vconnected to aproperly chosen dropping resistor 298 and then to terminal 300 whichconstitutes they positive output as present on the lead 302. A Zenerdiode 304 n is connected between the negative terminal 292 and thepositive terminal 300 for the'purpose of output'amplitude regulation.The breakdown voltage of thisA Zener diode would be a matter of designchoice but to cite an example, onevform of the apparatus in present useemploys a 2S volt regulation.

which is connected to a point 324,

It should be restated that the power supply 278 provides a full-waverectified output and that it is unfiltered. Hence, the output voltage isa series of plateaus and the output voltage value drops completely tozero at each half cycle. This intermittent D-C potential source isimportant to the proper operation of the circuitry as the periodic zerovoltage values serve to synchronize the conduction of unijunctiontransistors 260 and 270 to provide a highly accurate phase indication.

In the circuit of unijunction transistor 260, the base number onecircuit is connected through the primary 262 of transformer 364 and thesecondary windings 310 a'nd 312 provide output voltages suitable forgating onf the SCR components (Silicon Controlled Rectiers) in the 0 v fl v l i 1" high current supply 214. The winding 310 is connected Thetype of control whlch this clrcultry exer'clses lover via lead 314 tothe gate electrode of SCR 316 and the winding 312 is connected via lead318 to the gate electrode ofv SCR 320. The opposite sides of each ofsecondary windings'310 and 312 are tied together to a lead 322 shortedto the output power supply terminal 212.

' Similarly, the unijunction transistor 270 energizes the primary 272 oftransformer 274 such that SCR control voltages are induced in thesecondary windings 326 and 328. Secondary winding 326 is connected bylead 330 to the gate electrode of an SC-R 332 and the secondary winding328 connects by lead 334 to the gate electrode of an SCR 336. In thisconnection of transformer 274 the remaining leads of secondary windings326 and 328 are tied together and connected on a lead 33810 a terminal340 which is shorted to the power supply output terminal 220. Thus, thealternate energization of transformers 264 or 274 by the respectiveunijunction transistor 260 or 270 will result in one or the otherdirection of current fiow from the high current power supply 214terminal point 324, a negative potential terminal which iscommon to thepower supply output terminal 2112. The center tap of secondary winding350 is connected by means of lead 352 to the other power supply outputterminal 220.

The connection for the transformer 348 is opposite to that oftransformer 346. Thus, the end leads of the center tapped secondarywinding 354 are each connected to the anodes of the SCRs 336 and 332,however, the cathode connections of SCRs 332 and 336 are connectedtogether and to the terminal 340 which, in turn, is common to the powersupply output terminal 220 of the high current power supply 214. Thecenter tap of secondary winding 354 is connected by lead 356 to theother output terminal 212.

In operation of the apparatus, a polarization curve is first derived forthe particular materials of metallic vessel 200 and the corrosivesolution 202 so that corrosion protection apparatus can be properlypreset. For example, for a 1020 mild steel-alkaline sulphide pulpdigester system such as that in FIGS. through 9, the initial set pointor control point voltage should be set within the cathodic currentdensity range 106 of FIG. 5. The potential control 230 would then be setto provide some control point voltage Vc in the range between +800millivolts and +880 millivolts. Thus, corrosion protection is effectedwhen the metal-electrolyte system is brought within this range. However,it should be remembered that the pulp digestion process takes place overa wide range of temperatures from about 150 F. (FIG. 5) to about 350 F.(FIG. 9). Thus, it may be necessary to shift the control point duringthe pulp digestion process in order to maintain continuous protectivepolarization. Probably one potential shift would be all that is requiredand that would fall near the end of the heating up portion of thedigestion process. FIG. 8 shows the cathodic current region at about 300F. and a shift in the potential range of the secondary cathodic currentregion is apparent. Thus, it would probably be desirable to shift thesetting of potential control 230 upward about 100- millivolts duringpassage of the process through a temperature range including 300 F.

Once the initial set point is set into potential control 230 themetallic vessel 200 can be charged with the alkaline sulphide digestionliquor or corrosive solution 202. This initial condition would exhibitbehavior concident to the polarization curve of FIG. 5, taken at thelower 150 F. temperature. It should be understood however that somedigestion processes are charged or started at 212 F. and, in this event,the polarization curve of FIG. 6 would provide a similar startingreference.

When solution 202 is first charged into the vessel 200, themetal-electrolyte system assumes its natural corrosion potential. InFIG. 5, this would pertain to the point 102 at about 950 millivoltsvessel potential whereat the vesselsolution is actively corroding. Thisvessel potential of 950 millivolts is sensed then by the calomelhalf-cell 206 and conducted as the reference voltage Vr on lead 226 tothe input of the impedance converter 224. The irnpedance converter 224provides a reference voltage output Vr on lead 228 for application tothe voltage differential detection circuitry. Also, the potentialcontrol circuit 230 provides the preset control point voltage Vc on lead230 for simultaneous application to the detection circuitry.

It should then be noted that the reference voltage Vr on leads 228 andthe control point voltage Vc on leads 230 are oppositely applied asbetween the input and output transistors of the two balanced pairs oftransistors. That is, the reference voltage Vr on leads 228 is appliedto the output transistor 234 of one balanced pair while being applied tothe input transistor 246 of the other balanced pair, and the controlvoltage Vc on leads 230 is applied to the input transistor 232 on theone balanced pair while being applied to the output transistor 248 ofthe other balanced pair. Thus, since the transistors 232, 234,246 and248 are of the type PNP, a more negative base signal applied to theoutput side of one or the other of the balanced pairs will causedifferential conduction. When the reference voltage Vr on the base oftransistor 16 234 is more negative than the control point voltage Vc onthe base of transistor 232, conduction is increased through transistor234 to lire the relaxation oscillator circuit of unijunction transistor260 as will'bedescribed below. Similarly, in the other balanced pair theopposite reaction is induced by applying the reference voltage Vr to thebase of the input transistor 246 while applying the' control pointvoltage Vc to the base of transistor 2481 In this instance, a morenegative control point voltage Vc will cause conduction sufiicient toenergizethe relaxation oscillator circuit comprising the unijunctionytransistor 270.

In the initially charged vessel-solution system,l the reference voltageVr of the metallic vessel 'is higher than the control point voltage Vc.Therefore, the balanced pair of transistors 246 and 248 first corneintoplay since the control point voltage Vc is more negative than thereference voltage Vr. Transistor 248` conducts an amount determined bythe difference of the reference voltage Vr and control point voltage Vcto energize, thel relaxation oscillator. The oscillation is effected byperiodic' cohduction of the unijunction transistor 270 as controlled bythe resistance-capacitance time of the capacitor 256 in series with theemitter-collector effective resistance of the transistor 248. Hence, asthe transistor 248 increases `in conduction, the emitter-collectorresistance decreases thereby shortening the resistance-capacitance timesuch that the unijunction transistor 270 fires at a faster rate. Thatis, it fires sooner in each half cycle of the applied power supplyvoltage (from power supply 278) to cause a pulse of current conductionthrough the primary 272 of transformer 274. This pulse voltage isapparent in the secondary windings 326 and 328 and is conducted onrespective leads 330 and 334 to the gate electrode of the respectiveSCRs 336 and 332 to gate them into conduction. Thus, with the SCRs 336and 332 conducting, the current flow is such that output terminal 220 isa positive terminal and output terminal 212 is negative. Therefore thereis anodic current flowing through metal-electrolyte system and thevessel 200 is the anode while the auxiliary electrode 204, wh-ich isconnected to the output terminal 212 of the power supply 214, is acathode.

At first, the anodic current will be caused to flow heavily since thecomparison of the reference voltage Vr and the control point voltage Vcare the greatest. This, in turn, will cause highest differentialconduction in transistor 248 and thus earlier firing of the unijunctiontransistor 270 in each half cycle of the energizing voltage from powersupply 278. Then as the anodic current flows and the vessel potentialmoves through the Flade arrest point 104 (FIG. 5) the potential of thevessel 200 is reduced thereby decreasing the error voltage or differencebetween the reference voltage Vr and the control point voltage' Vc'.This, of course, allows the unijunction transistor 270 to be firedprogressively later in each half cycle of the power application tothereby fire the SCR members 332 and 336 later in each half cycle, thusapplying less average power to the vessel-solution system. It should benoted that the power supply 278, mentioned previously as'beingunfiltered, plays an important part in the synchronization of thesystem. It provides positive and negative supply voltages which drop tozero on each half cycle thereby synchronizing or providing a regular andreliable starting point for the R-C combination, transistor 2'48 and thecapacitor 256 of the relaxation oscillator (unijunction transistor 270).

As the vessel potential decreases through the-primary anodic currentregion containing the Flade arrest 104 (FIG. 5) and passes into thecathodic current density region 106 (FIG. 5), the vessel potential willarriver at thev preset control point voltage Vc. In this condition, thereference voltage Vr and the control voltage Vc will be equal and nocorrosion protection power application is effected. Since this cathodiccurrent density region is a stable vessel potential more noble than theFlade arrest potential, it is 17 desirable to maintain the-potentialwithin this region of secondary cathodic current. When the vesselpotential, as detected by the reference voltage Vr, drifts toward a morenoble or less positive voltage than y thatl of the controlpoint voltageVc, the other balanced pair of transistors 232 and 234 and the'respective relaxation oscillator, unijunction transistor 260,=will.be.energized to effect control. The reference .voltage Vr, beingmore negative than the control point voltage Vc,

-reduces 4the emitter-collector,resistance of Atransistor234 therebycausing Vconduction and-,energization of the other relaxationoscillator, :unijunction transistor 260. As before, the timing capacitor244 and the transistor 234-act to time .the firing. point of theunijunction transistor 260 such that .increasing Vconduction advancesthe firing point in each half cycle Of operation-as controlled by theunfiltered power supply 278. Upon tiring of the unijunctiontransistor260, the .transformer 264 provides induced firing pulses in secondarywindings 310 and 312 which-are applied on leads 314 and 322 to therespective gate electrodes of SCRs 316 `and 320. Whenthese SCRs 316 `and320 ,are

gated on the current flow is opposite to that produced previously bygating on the SCRs332 -and 336. That-is, current flow is from-the- SCRs316 and 320 to the intermediate terminal 324 and output terminal 212 andthen on lead 21010 the auxiliary electrode 204. The return being from,the metallicvessel 200 through the lead 218 and back to the outputterminal 220 of the high current power supply 2 14. In this applicationthe applied current is cathodic in character, .that is, with respect tothe metal-electrolyte V,pulp digestion process, the potential control230 would require several .settings during the heat up and cookingprocess. It should be understood, however, that each metalelectrolytesystem Will have its own specialized behavior .characteristics and thatthe flexibility of control afforded by the present apparatus is a verydesirable feature.

It has been found that some systems can be brought through the Fladepotential to a more noble, non-corrosive vessel potential and that theywill remain in that condition for a longy period of time with no furtherpotential variation. In the event that there should be a vesselpotentialchange, however,y the control `apparatus will apply either thevanodic current or cathodic current to counteract the potential changeand bring it back to the set point -or pre-determined potential controlvoltage. In most sys- .tems there will be a drift of the vesselpotential in one directionfor the other after it has been initiallybrought to the stable, non-corrosive potential. This then requiresperiodic current application to the vessel-solution system, eitheranodic or cathodic, and the apparatus of FIGS. l() and 11 serve toprovide the current flow of proper polarity and amounts in response toan error voltage as detected with respect to the actual vesselpotential.

It should be understood that there are other equivalents which could besubstituted for thespecific components of the apparatus disclosedherein. It is contemplated that a high current power supply may beconstructed using rectifier elements of the gate turn-off type such thateither anodic or cathodic current could be applied through thevessel-solution system with highly accurate regulation.

While only a few particular metal-electrolyte systems have beendisclosed with respect to the present method and apparatus, it isforeseen that there will be a great many metal-corrosive solutioncombinations which will exhibit the special polarization behavior. Themethod and 'apparatus of the present invention will be applicable to al1of such systems and, due to its inherent versatility, the method landapparatus can be employed inconventional corrosion protectionapplications as well. That is, the apparatus may be used -with allmetal-electrolyte systems exhibiting behavior which includes thepresence in the polarization curve of stable current densities, eitheranodic or cathodic, at potentials lying between the Flade arrest and thetranspassive potential region. .f f

Changes may be made in the combination and arrangement of elements andsteps as heretofore setforth in this specication and shown in thedrawings; it being understood, that changes may be made'in theembodiments disclosed without departing from the spirit and scope oftheinvention as defined in the following claims. l'

What is claimed is: v 1. A method of corrosion protection for. a.metallic vessel containing a corrosive solution, said metallic vesseland corrosive solution system having ay polarization curve whereinincreasing noble vessel potentials are characterized by anodic currentdensity through the Flade arrest potential and then cathodic currentdensity, comprising the steps of:

applying direct current `energy between the metallic vessel and anelectrode immersed in said corrosive solution; and v controlling thepolarity of said applied direct current energy in accordance with thepotential of the metallic vessel.

2. A method of corrosion protection for a metalcorrosive solutionsystem, said system having both anodic and cathodic current regions inthe polarization curve, comprising the steps of:

monitoring the potential of the metal;

passing direct current energy between the metal and the corrosivesolution; and controlling the polarity of said direct current energy inaccordance with said potential of the metal.

3. A method of corrosion protection for'a metallic vessel containing acorrosive solution comprising the steps of:

monitoring the potential of the metallic vessel;

passing direct current between the metallic vessel and an electrodeimmersed in said corrosive solution; and controlling the polarity of thedirect current with said monitored potential such that anodic currentflows at initial, actively corroding vessel potentials and cathodiccurrent fiows at a more noble, non-corrosive vessel potential.

4. A method of corrosion protection for a metallic vessel containing acorrosive solution, said metallic vessel and corrosive solution systemhaving a polarization curve wherein the system shows a minimum currentregion of cathodic current density at vessel potentials more noble thanthe Flade arrest potential, comprising the steps of:

applying direct current energy between the metallic vessel and anelectrode immersed in said corrosive solution; and

controlling the polarity and amount of applied direct current energy inaccordance with the polarity and amount of potential difference detectedbetween said metallic vessel and a reference electrode immersed in saidsolution.

5. A method as set forth in claim 4 wherein:

positive direct current energy is applied to the vessel upon detectionof first predetermined indications by said reference electrode; and vnegative direct current energy is applied to the vessel upon detectionof second predetermined indications by said reference electrode.

6. A method of controlled potential corrosion protection in metallicvessel-corrosive solution systems which exhibit special polarizationbehavior comprising the steps of:

applying anodic current between the metallic vessel and an electrodeimmersed in said solution at vessel potentials which are more activethan a predetermined voltage; and

applying cathodic current between said vessel and electrode immersed insaid solution when the vessel potential is more noble than apredetermined voltage.

7. A method as set forth in claim 6 wherein said metallicvessel-corrosive solution comprises:

metal-electrolyte systems which exhibit a vessel potential range ofcathodic current density at vessel potentials more noble than the Fladearrest potential.

8. A method as set forth in claim 7 wherein said metallicvessel-corrosive solution system comprises:

stainless steel and a hot, concentrated alum solution.

9. A method as set forth in claim 7 wherein said metallicvessel-corrosive solution system comprises:

Carpenter 20 steel and a sulphuric acid/zinc solution.

10. A method as set forth in claim 7 wherein said metallicvessel-corrosive solution system comprises:

carbon steel and an alkaline sulphide liquor. 11. A method as set forthin claim 6 wherein said metallic vessel-corrosive solution comprises:

metal-electrolyte systems which exhibit a vessel potential range ofcathodic current density at vessel potentials more noble than the Fladearrest potential; and

said systems exhibit a range of anodic current density at vesselpotentials still more noble than said cathodic current vessel potentialrange.

12. Apparatus for corrosion protection of a metallic vessel containing acorrosive solution, said metallic ves sel and corrosive solution systemhaving a polarization curve wherein the vessel and solution shows aminimum cathodic current region at vessel potentials more noble than theFlade arrest potential, comprising:

electrode means in contact with said solution;

direct current means connected between said electrode means and saidmetallic Vessel; and

automatic means for applying positive direct current to said vessel tocontrol the current density when it is anodic and for applying negativedirect current to control the current density when it is cathodic.

13. Apparatus for controlled potential corrosion protection of ametal-corrosive solution system comprising:

a source of direct current energy;

means for applying said direct current energy between the metal and anelectrode immersed in said corrosive solution;

means for sensing the potential of the metal; and

automatic means for controlling the amount and polarity of said directcurrent energy in accordance with said sensed potential.

14. Apparatus for corrosion protection of a metallic vessel containing acorrosive solution, said vessel-solution system having a polarizationcurve wherein a low cathodic current density exists at vessel potentialsmore noble than the Flade arrest potential, comprising:

electrode means in contact with said solution;

direct current means connected between said electrode means and saidmetallic vessel;

reference electrode means in contact with said solution and electricallyconnected to said metallic vessel; and

automatic means controlling said direct current means to apply negativecurrent to said vessel in response to a first predetermined range ofvoltages as detected between said vessel and said reference electrodemeans and to apply positive direct current to said vessel in response toa second predetermined range of voltages as detected between said vesseland said reference electrode means. l 15. Apparatus for corrosionprotection of a metallic vessel containing a corrosive solution, saidvessel-solution system having a polarization curve wherein a region oflow cathodic current density exists at vessel potentials more noble thanthe Flade arrest potential, comprising:

electrode means immersed in said solution;

a direct current source connected between said electrode and saidvessel;

reference electrode means in electrolytic contact with said solution,and electrically connected to said vessel to detect its potential;

automatic means for comparing the potential detected by said referenceelectrode with a reference potential to develop Ea control voltage; and

automatic means connected to receive the control voltage and regulatethe polarity and amount of direct current which is applied from saiddirect current source.

16. Apparatus for controlled potential corrosion protection of ametallic vessel-corrosive solution which exhibits special polarizationbehavior, comprising:

a direct current source;

an electrode adapted to contact the solution;

means for applying direct current between the metallic vessel and saidelectrode;

means for sensing the potential of the metallic vessel;

and

automatic means responsive to said sensed potential for controlling saidmeans for applying so that anodic current flows at vessel potentialsmore active than the Flade arrest potential and cathodic current ows atpotentials more noble than the Flade arrest potential.

17. Apparatus for controlled potential corrosion protection of metallicvessel-corrosive solution systems which exhibit special polarizationbehavior, comprising:

a source of direct current energy;

an auxiliary electrode immersed in said solution;

a reference electrode in contact with said solution and electricallyconnected to said metallic vessel for sensing a reference potential;

means for providing a predetermined control point voltage; and

automatic means for applying anodic direct current between said metallicvessel and auxiliary electrode when said reference potential is moreactive than said control point voltage, and for applying cathodic directcurrent when said reference potential is more noble than said controlpoint voltage.

18. Apparatus for corrosion protection of a metallic vessel containing acorrosive solution, comprising:

electrode means immersed in said solution;

a direct current source having its output connected between saidelectrode and said metallic vessel;

a reference electrode immersed in said solution and electricallyconnected to said metallic vessel;

automatic means for comparing the voltage between said metallic vesseland said reference electrode with a reference voltage to develop a firstcontrol voltage when said reference voltage differs in a first polarityand a second control voltage when said reference voltage differs in theopposite polarity; and

automatic means responsive to said rst and second control voltage forrespectively applying positive and negative polarities of direct currentfrom said source to the vessel. 19. Apparatus for controlled potentialcorrosion protection of a metallic vessel containing a corrosivesolution comprising:

an auxiliary electrode immersed in said corrosive solution;

a direct current power source connected between said metallic vessel andsaid auxiliary electrode;

means for sensing the potential of the metallic-vessel;

automatic means for comparing the sensed vessel potential with apredetermined control point potential to derive a control voltage; and

automatic means controlled by said control voltage for varying theamount and polarity of the output of said direct current source.

means for providing a predetermined control point voltage;

automatic means forcomparing said vessel potential and control pointvoltage to derive an error voltage; and v automatic means conductive inresponse to said error voltage to regulate the amount and polarity ofdirect current from said sou'rce through said metallic vessel, corrosivesolution and auxiliary electrode.

21. Apparatus as set forth in claim 20 wherein said means conductive inresponse to said error voltage comprises:

a PNP transistor having the collector connected to the negative terminalof said direct current source, the emitter connected tosaid auxiliaryelectrode, and the lbase connected to receive said en'or voltage;

an NPN transistor having the collector connected to the positiveterminal of said direct current source, the emitter connectedto saidauxiliary electrode, and the base connected to receive said errorvoltage; and

a common, mid-potential of said direct current source -being connectedto said metallic vessel.

22. Apparatus as set forth in claim 20 wherein said means for comparingcomprises:

a first differential amplifier receiving the reference potential and themore noble control point voltage at respective inputs to derive an erorrvoltage at the output; and

a second differential amplifier receiving the control point voltage andthe more noble reference potential at respective inputs to derive anerror voltage at the output. t y

23. Apparatus as set forth in claim 22 wherein said means conductive inresponse to said error voltage comprises:

a first relaxation oscillator conductive in response to an error outputvoltage from said first differential amplifier to cause a first polarityof output from said direct current source; and

a second relaxation oscillator conductive in response to an error outputvoltage from said second differential amplifier to cause a secondpolarity of output from said direct current source.

24. Apparatus as set forth in claim 23 wherein said direct currentsource comprises:

an A-C voltage source;

a first pair of semi-conductor controlled rectifiers connected to saidA-C source for full-wave rectification;

a second pair of semi-conductor controlled rectifiers connected inparallel to said A-C source for 'fullwave rectification and in reversedpolarity to said first pair of said rectiers;

means for gating said first pair of rectifiers into conduction duringthe conduction of said first relaxation oscillator; and

means for gating said second pair of rectifiers into conduction duringthe conduction of said second relaxation oscillator.

25. Apparatus for controlled potential corrosion protection of ametallic vessel containing a corrosive solu- Cil tion, wherein thevessel-solution system exhibits special polarization behavior,comprising:

an auxiliary electrode immersed in said corrosive solution;

a reference electrode in electrolytic contact with said corrosivesolution and electrically connected to said metallic vessel for derivinga reference potential;

potential control means for providing a predetermined control pointpotential;

a direct current power supply having negative, positive and commonoutput terminals, said common terminal being connected to said metallicvessel;

a PNP transistor having the collector connected to said negative powersupply terminal and the emitter connected to said auxiliary electrode;

an NPN transistor having the collector connected to said positive powersupply terminal Aand the emitter connected to said auxiliary electrode;and

a voltage comparator receiving said reference potential and controlpoint potential to derive an error voltage the output of which isconnected in parallel to the rbases of the PNP and NPN transistors.

26. Apparatus for controlled potential corrosion protection of ametallic vessel-corrosive solution system which exhibits specialpolarization behavior, comprising:

an auxiliary electrode immersed in said corrosive solution;

areference electrode in electrolytic contact with said corrosivesolution for deriving a reference potential;

potential control means for providing a predetermined control pointpotential;

differential amplifier means for providing a first error voltage whensaid reference potential exceeds said control point potential and asecond error voltage when said reference potential is more negative thansaid control point potential;

an alternating-current voltage source;

a first pair of semi-conductor controlled rectifiers connected to saidalternating-current voltage source as a full-wave rectifier with thepositive output terminal connected to the metallic vessel and thenegative output terminal connected to said auxiliary electrode;

a second pair of semi-conductor controlled rectifiers connected inparallel with said first pair and connected to saidalternating-currentvoltage source as a full-wave rectifier with thenegative output terminal connected to the metallic vessel and thepositive output terminal connected to said auxiliary electrode;

a first relaxation oscillator intermittentlyconductive for a durationdetermined -by said firste'rror voltage to gate-on said first pair ofsemi-conductor controlled rectifiers to apply anodic current throughsaid metallic vessel and corrosive solution; and

a second relaxation loscillator intermittently conductive for a durationdetermined `by said second error voltage to gate-on said second pair ofsemi-conductor controlled rectifiers to apply cathodic current throughsaid metallic vessel and corrosive solution.

27. Apparatus as set forth in claim 26 which is further characterized toinclude:

a low voltage, unfiltered power supply which drops to zero in each halfcycle for synchronously energizing said first and second relaxationoscillators.

No references cited.

HOWARD S. WILLIAMS, Primary Examiner.

T. TUNG, Assistant Examiner.

1. A METHOD OF CORROSION PROTECTION FOR A METALLIC VESSEL CONTAINING ACORROSIVE SOLUTION, SAID METALLIC VESSEL AND CORROSIVE SOLUTION SYSTEMHAVING A POLARIZATION CURVE WHEREIN INCREASING NOBLE VESSEL POTENTIALSARE CHARACTERIZED BY ANODIC CURRENT DENSITY THROUGH THE FLADE ARRESTPOTENTIAL AND THEN CATHODIC CURRENT DENSITY, COMPRISING THE STEPS OF:APPLYING DIRECT CURRENT ENERGY BETWEEN THE METALLIC VESSEL AND ANELECTRODE IMMERSED IN SAID CORROSIVE SOLUTION; AND CONTROLLING THEPOLARITY OF SAID APPLIED DIRECT CURRENT ENERGY IN ACCORDANCE WITH THEPOTENTIAL OF THE METALLIC VESSEL.